This invention relates to a method and apparatus for preparing optical information media such as read-only optical disks and optical recording disks.
To record and store a vast quantity of information as typified by moving image information, advanced optical information media such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion to
xcex/(dxc2x7NA3) 
wherein xcex denotes the wavelength of Recording/reading beam and xe2x80x9cdxe2x80x9d denotes the thickness of a transparent substrate the Recording/reading beam enters. If the optical recording medium is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(xc2xd)xc2x7dxc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2xcex8)xe2x88x92{fraction (5/2)}
wherein n denotes the refractive index of the substrate and xcex8 is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness xe2x80x9cdxe2x80x9d of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 0.1 mm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a greater NA because the substrate can be made extremely thin as compared with the prior art. Media having such structure are disclosed in JP-A 10-320859 and 11-120613.
The provision of a light-transmitting layer of approximately 0.1 mm thick allows for use of an objective lens having a large numerical aperture NA, say about 0.85.
The light-transmitting layer of approximately 0.1 mm thick can be formed, for example, by a spin coating technique. The spin coating technique generally involves feeding a resin onto the surface of a disk substrate secured on a rotating table, and rotating the disk substrate for spreading the resin by a centrifugal force. Since the disk substrate is formed with a center hole which is utilized for mounting the disk on a drive, the resin cannot be fed to the center of rotation (or the center of the disk substrate), but to an annular band equidistantly spaced apart from the center of rotation. As the position of resin feed is spaced apart from the center of rotation, the resulting coating or light-transmitting layer has a greater thickness variation in a radial direction.
To reduce the radial thickness variation of the light-transmitting layer, several proposals were made to close the center hole of the disk substrate with suitable plug means such as a plate member, disk member, plug or cap so that the resin can be fed near the center of the plug means, that is, the center of rotation. See JP-A 10-320850, 10-249264, 10-289489, 11-195250, and 11-195251.
However, the techniques of JP-A 10-320850, 10-249264, and 11-195250 are difficult to practice in the industry because it is not described how to remove the plug means such as the plate member or cap at the end of spin coating.
In contrast, JP-A 10-289489 describes that at the end of spin coating, the plug means such as the disk member is removed by punching out or magnetic attraction using an electromagnet. However, the punching step or the removal by magnetic attraction is a cumbersome operation. The plug means is removed at a high acceleration, causing a disturbance to the resin coating.
The above-referenced JP-A 11-195251 describes a plug means of the structure having a circular cap and an integral support at the center thereof. The support allegedly facilitates attachment/detachment and alignment of the plug means. The support described therein is a hollow cylinder having at least one discharge port or consists of a plurality of rods. Once a resin is introduced into the interior of the hollow cylinder or the region circumscribed by the rods, the disk substrate is rotated together with the plug means whereby a resin layer is formed on the disk substrate. Finally the plug means can be readily removed.
With this plug means used, spin coating is carried out by causing the resin to flow out of the discharge port in the hollow cylinder or the gaps between the rods. Therefore, the resin is retained or dammed by the support wall (region other than the discharge port) or the rods. The once dammed resin can flash over the disk substrate at incidental timing, frequently forming asperities on the coating. Also the side of the plug means to be in contact with the resin has a complex shape and a large contact area, which is inconvenient at washing the plug means. If some resin is left on the surface of the plug means, the coating frequently becomes uneven. Table 1 of the above-referenced JP-A 11-195251 reports the thickness variations of the coating formed when the outer diameter of the hollow cylinder ranges from 4 mm to 16 mm. It is evident from the test results that the thickness variation of the coating depends on the outer diameter of the hollow cylinder. The larger the outer diameter, the greater becomes the thickness variation. More specifically, even if the resin is fed to the interior of the hollow cylinder, the coating start position is not coincident with the center of rotation, but the outer perimeter of the hollow cylinder. With the relatively high viscosity of the resin taken into account, it is difficult to reduce the outer diameter of the hollow cylinder below 4 mm. The method of this patent is thus very difficult to minimize the thickness variation of the resin coating.
An object of the invention is to provide a method and apparatus for preparing an optical information medium comprising a disk-shaped supporting substrate, an information recording layer thereon, and a resin-based light-transmitting layer thereon by which recording/reading laser beam is transmitted, the method and apparatus being capable of minimizing the thickness variation of the light-transmitting layer without significant complication of the method and apparatus.
In one aspect, the invention provides an apparatus for preparing an optical information medium which comprises a disk-shaped supporting substrate having a center hole, an information recording layer thereon, and a resin-based light-transmitting layer on the information recording layer by which recording/reading laser beam is transmitted. The apparatus includes a rotating table for holding and rotating the supporting substrate having the information recording layer borne thereon. A plug means for closing the center hole includes a disk member for closing the center hole and an integral support shaft extending from the disk member at the center thereof. A feed means is provided for feeding a coating fluid containing the resin to the outer periphery of the support shaft.
In one preferred embodiment, at least a portion of the support shaft is a frustoconical portion whose diameter gradually decreases toward the disk member.
In another aspect, the invention provides a method for preparing the optical information medium defined above using the apparatus defined above. The method involves the steps of mounting the supporting substrate having the information recording layer borne thereon on the rotating table; closing the center hole of the supporting substrate with the disk member of the plug means; feeding the coating fluid from the feed means to the outer periphery of the support shaft, then onto the disk member of the plug means; rotating the supporting substrate together with the plug means, whereby the coating fluid is spread from the disk member over the supporting substrate to form a resin layer; and curing the resin layer into a light-transmitting layer.
The preparation apparatus of the invention includes, as shown in FIGS. 1 and 3, a rotating table 2 for holding and rotating a disk substrate 100 (supporting substrate having an information recording layer borne thereon) having a center hole 101, plug means 3 for closing the center hole 101, and feed means in the form of a nozzle 4 for feeding a coating fluid 5 containing the resin. The plug means 3 includes a disk member 31 for closing the center hole 101 and an integral support shaft 32 extending from the disk member 31 at the center thereof. The nozzle 4 feeds the coating fluid 5 to the outer periphery of the support shaft 32.
The provision of the support shaft 32 in the plug means 3 facilitates the handling of the plug means 3 during the medium preparation process and especially, the removal of the plug means 3 at the end of spin coating.
As previously discussed, JP-A 11-195251 describes a plug means of the structure having a circular cap and an integral support in the form of a hollow cylinder or a plurality of rods. The plug means used in the present invention has the following advantages over the plug means of this patent reference.
In JP-A 11-195251, the resin is dammed by the support wall or rods, which can cause thickness variations to the coating. In contrast, according to the present invention, the coating fluid is fed to the outer periphery of the support shaft, followed by spin coating. This sequence minimizes the thickness variation of the coating. Since the resin is applied to the outer periphery of the support shaft, washing of the plug means is easy as compared with the plug means of JP-A 11-195251. Since a coating fluid having a relatively high viscosity is fed to the interior of the hollow cylindrical support in JP-A 11-195251, the outer diameter of the support cannot be reduced below the limit necessary to maintain the coating fluid flowable. As a result, the coating start position is located relatively far from the center of rotation. In contrast, the present invention allows the outer diameter of the support shaft to be significantly reduced, which also contributes to the minimization of thickness variation of the coating.